Aphids continue to cause debate

AgriLand received a number of enquiries about aphid numbers and spraying decisions on spring crops this week. There is a lot of interest in how these little insects, which spread barley yellow dwarf virus (BYDV), affect crops.

To answer some of the questions, we’ve pulled together some of the main points made by Teagasc entomologist Louise McNamara at the Irish Tillage and Land Use spring workshop.

Aphid numbers – reported by Teagasc – for the last week of April were at 4.1 aphids/m². This compares to 266.5 aphids/m² in the same week last year.

Numbers are low; but Teagasc is advising farmers to still spray. A pyrethroid insecticide should be sprayed on spring barley at GS14 (the four-leaf stage).

The grain aphid

The grain aphid is the most important aphid affecting cereal crops in this country and can vary in colour (yellow, green, reddish brown or black). These aphids affect yield and quality through feeding and viral transmission.

Louise explained: “The grain aphid is always on cereals and grasses. A small proportion of these aphids over-winter as eggs; but the majority over-winter as live aphids on grasses and cereals. They can rapidly develop once temperatures rise.

They don’t have alternative hosts and fresh migrations infest crops from April.

“Grain aphids directly feed from April to August and they can spread BYDV, primarily from September until March. It’s of most concern to the crop pre GS31.

“The grain aphid is more cold hardy than other species. It can survive down to -8°C. That’s why it doesn’t lay eggs as frequently as the others,” Louise added.

In the past, they were well controlled using pyrethroids; but in recent years – due to control failures – we’ve detected KDR, which is knockdown resistance or partial resistance to pyrethroids.

“If you plant your crop later, it is at the vulnerable stages when aphids are migrating. If you plant your crop earlier in March, it has reached a later stage in development and the aphids will have less of an impact on yield when the aphids get into the crop.”

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Aphid flight

According to Louise, aphid flight conditions are optimum at 14-15°C (factors other than temperature will also affect this). However, movement can happen in sub-optimum conditions. This can include walking on the crop or short distance flight.

“In most cases BYDV is spread by winged aphids migrating from an infected source outside of the field,” she explained.

BYDV damage

Previous research by Tom Kennedy in Teagasc showed that yield losses can be up to 80%, with an average of around 30%. Yield loss can be up to 3.7t/ha in winter barley and almost 2t/ha in spring barley.

Symptoms of BYDV include:
  • Phloem degradation;
  • Leaf discolouration;
  • Plant stunting;
  • Less grains per ear;
  • Grain yield reduction;
  • In severe cases, death of the plant.

Risk

The risk of BYDV infection is greater in mild winters. Louise explained that the longer it is mild for, the longer the aphids are active. Spring crops after mild winters are at risk.

Resistance to pyrethroids

Over 30 insect species have developed resistance to pyrethroids. Aphids with the knockdown resistance (KDR) gene are less susceptible to pyrethroids.

Louise explained that resistant aphids have been found all across the country. However, not all aphids are resistant to pyrethroids.

“To date, KDR has only been identified in the grain aphid, which is our most prevalent aphid. In the UK and Ireland there is one group that is resistant. Aphids carrying this resistance gene are found in all major grain growing regions in Ireland; everywhere we tested, we found resistant aphids.”